Allow custom sanitize function

[adamb70]

As per markedjs/marked#1519, markedjs is deprecating their sanitizer options. EasyMDE currently doesn't allow any other options for sanitizing the markdown preview.

They are suggesting using an external sanitization library such as DOMPurify (recommended), sanitize-html or insane. As they have slightly different API's for handling their options I suggest simply providing the option to wrap the markedjs HTML output in a custom sanitizer function, which can accommodate any external library.

function(renderedHTML) {
	// Using DOMPurify with options
	return DOMPurify.sanitize(renderedHTML, {ALLOWED_TAGS: ['b']})
}

[Ionaru]

Code looks good, can you add this to the changelog as well?

[A-312]

@Ionaru done

[Ionaru]

Does the typeof check work with arrow functions as well?

const sanitizer = () => ...;

[Ionaru]

Tested, it should work.